The role of surface tension lowering by organics in the formation of cloud droplets

The reflectance of clouds is dependent, in part, on the number concentration of cloud droplets and therefore indirectly on the availability of so-called cloud condensation nuclei (CCN). Our new study highlights the interplay of organics, liquid-liquid phase separation and presence of ultrafine aerosols in determining the CCN number concentration.

 

Every liquid water droplet present in a (ice-free) cloud started out as a tiny aerosol particle in the air – one out of the class of those particles also known as cloud condensation nuclei (CCN). CCN have favourable physical and chemical properties which allow them to undergo a rapid growth process by condensation of water vapour when the relative humidity in the air is at least slightly above 100 % (known as water supersaturation). The conditions at which such an aerosol particle starts the rapid growth into a small cloud droplet is known as CCN activation, since a thermodynamic energy barrier must be overcome to initiate the process. The necessary environmental conditions in the air and many features of the CCN activation and cloud droplet formation process have been well understood; however, the diverse effects of organic matter, typically present in mixed organic-inorganic aerosol particles, has been subject to much debate. On the one hand, organic compounds, especially those of low water-solubility, may contribute little to the hygroscopic growth (the water uptake of particles via the solute effect) and could therefore be a drawback for the cloud droplet formation potential (on a per dry volume or dry size basis of particulate matter). On the other hand, organics may contribute to a lowering of the droplet surface tension if present/enhanced near the surface – a typical feature of surfactants. The droplet surface tension is a key property affecting the CCN activation potential.  

An international team of US, Canadian, Italian, Finnish, French and Irish-based researchers, including Prof. Andreas Zuend from the AOS department at McGill, have conclusively found that organic compounds can increase the activation efficiency of cloud nuclei into cloud droplets via a liquid-liquid phase separation mechanism, ultimately increasing the cooling effect of maritime clouds.  It is the first team to confirm this hypothesis under natural environmental conditions, based on experimental and theoretical work.

The team, led by Prof. Colin O’Dowd from the School of Physics and Centre for Climate and Air Pollution Studies at NUI Galway, Ireland, deployed sophisticated aerosol and CCN activation measurement equipment to quantify this effect for the first time and developed the theoretical framework to underpin the experimental findings.  Using the most advanced thermodynamic aerosol droplet models, the team simulated the cloud droplet activation process using mixed organic-inorganic CCN and found that liquid-liquid phase separation could occur even for a small enrichment of organic surfactants without substantially changing the solute effect. The surfactants, once concentrated at the droplet surface in an organic-rich phase, readily lower surface tension, as the organic shell phase is separated from the inorganic solution phase in the core of the droplet. Observations taken during field measurements at the Mace Head atmospheric research station located at Ireland’s west coast, show that this organic effect can increase the availability of cloud condensation nuclei by up to a factor of 10.

This work suggests that the existence of certain cloud condensation nuclei in a liquid-liquid phase-separated state is not just a phenomenon of theoretical interest; it comes with real consequences. When the conditions in the environment are right, as they were during our field study at Mace Head, there are important implications for cloud formation and microphysical cloud properties.

The findings have been published as a letter in the top scientific journal Nature (titled “Surface tension prevails over solute effect in organic-influenced cloud droplet activation”, doi:10.1038/nature22806).

About the author
Prof. Andreas Zuend of McGill University’s Department of Atmospheric and Oceanic Sciences, the study’s second author, has been a leader in the development of predictive thermodynamic models for complex organic-inorganic mixtures such as those relevant for atmospheric aerosols and cloud droplets. Group website

Reference

Ovadnevaite, J., Zuend, A., Laaksonen, A., Sanchez, K. J., Roberts, G., Ceburnis, D., Decesari, S., Rinaldi, M., Hodas, N., Facchini, M. C., Seinfeld, J. H., and O’Dowd, C.: Surface tension prevails over solute effect in organic-influenced cloud droplet activation, Nature, 546, 637–641, doi:10.1038/nature22806, 2017.